Method for producing a fuel gas containing hydrogen for electrochemical cells and associated device

ABSTRACT

The invention relates to a process and an apparatus for producing hydrogen-containing fuel gases for fuel cells by catalytic reforming of hydrocarbons and subsequent gas purification. The process is characterized in that the catalytic reforming comprises two successive stages of which the first stage comprises autothermal reforming and the second stage comprises low-temperature steam reforming at temperatures below 650° C. In the first stage (autothermal reforming, ATR stage), a feed mixture of hydrocarbons, oxygen and water or water vapour is reacted over a catalyst in an autothermal reforming reaction to convert it incompletely into a hydrogen-rich gas mixture. The mixture which still contains residual amounts of hydrocarbons is then reacted in a subsequent steam reforming stage (second stage, SR stage) to give a hydrogen-rich fuel gas. A fuel gas which has a temperature at the reactor outlet of 400-650° C. and contains a very high proportion of hydrogen is obtained. Owing to the low outlet temperatures, the fuel gas can be passed directly to a gas purification stage without use of additional heat exchangers. In addition to the improvement in the reformer efficiency, a more compact and cheaper reformer design is made possible by the invention. Process and apparatus are used for producing hydrogen or hydrogen-containing fuel gases for fuel cells, in particular for mobile and stationary applications.

The present invention relates to a process for producing fuel gases forfuel cells. Here, a hydrogen-containing fuel gas is produced byreforming of hydro-carbons and is purified in further process steps.Furthermore, an apparatus for carrying out this process is described.

The process of the invention for producing hydrogen-containing fuelgases is based on a multistage reforming of hydrocarbons and asubsequent purification of the fuel gas by means of downstream reformatepurification processes. These can, for example, be based on a water gasshift reaction (WGS reaction) or on a gas separation membrane.

The reforming of hydrocarbons according to the invention is a two-stageprocess and comprises an autothermal reforming and a downstream steamreforming. In the first stage, a feed mixture of hydrocarbons, air andwater or water vapour is reacted over a catalyst in an autothermalreforming reaction to convert it incompletely into a hydrogen-rich gasmixture. This mixture, which still contains residual amounts ofhydrocarbons, is then reformed in a subsequent steam reforming stage togive a hydrogen-rich fuel gas. A fuel gas which has a temperature at thereactor outlet of from 450 to 650° C. and contains a high proportion ofhydrogen is obtained. The apparatus for reforming (the reactor) isconstructed as a two-stage reactor, with a different catalyst being usedin each stage. The fuel gas is subsequently subjected directly tofurther purification, for example in a water gas shift reactor or bymeans of gas separation membranes. Process and apparatus are used forproducing hydrogen-containing fuel gases for fuel cells, in particularfor mobile applications but also for stationary applications.

It is known that hydrogen can be produced by reacting hydrocarbons atelevated temperature in the presence of water vapour over a suitablecatalyst to form hydrogen, carbon monoxide and carbon dioxide. Thisreaction, also referred to as “steam reforming” (SR), is stronglyendothermic and proceeds, for example, according to the followingreaction equation:C₈H₁₈+8H₂O⇄8CO+17H₂ ΔH=+1250 kJ/mol  (1)

A characteristic parameter for the steam reforming reaction (1) is thesteam/carbon ratio S/C. In the reaction equation (1), S/C is equal to 1.Owing to the endothermic nature of this reaction, heat has to besupplied. However, if no heat is supplied (i.e. the reaction is carriedout adiabatically), the reaction takes the heat required from theenvironment, so that a decrease in the temperature of the overall systemoccurs. This principle is utilized in the present invention.

A further known possible way of producing hydrogen is catalytic partialoxidation (CPO). Here, the hydro-carbons are reacted in the presence ofoxygen over a catalyst according to the reaction equation for thepartial oxidation (2) to form carbon monoxide and hydrogen:C₈H₁₈+4O₂⇄8CO_((g))+9H₂ ΔH=−685 kJ/mol  (2)

An important parameter for the partial oxidation is the air index λ,which is defined as the ratio of the number of moles of oxygen used tothe number of moles of oxygen required for total oxidation [cf. reactionequation (3)]:C₈H₁₈+12.5O₂⇄8CO₂+9H₂Oλ=1 ΔH=−5102 kJ/mol  (3)

Complete conversion of the hydrocarbon into carbon monoxide and hydrogenin accordance with equation (3) requires an air index λ of <1, ideallyλ=4/12.5=0.32.

Autothermal steam reforming (“autothermal reforming”, ATR) consists oftwo part processes. It combines the steam reforming of equation (1) withthe catalytic, partial oxidation of equation (2), with the exothermic,partial oxidation supplying the heat of reaction necessary for theendothermic steam reforming. The feed mixture can be preheated to apreheating temperature. The product mixture is in thermodynamicequilibrium at the temperature prevailing at the reactor outlet.Autothermal reforming combines the advantages of the catalytic, partialoxidation (good starting behaviour) with those of steam reforming (highhydrogen yields) and is therefore preferably used for producing hydrogenin mobile fuel cell systems by means of on-board reforming. In thepresent patent application, autothermal reforming is regarded as asingle process step despite the fact that it consists, as described, oftwo part processes.

EP 0 112 613 B1 describes a process for the autothermal reforming ofhydrocarbons in which the partial oxidation occurs in zone 1 and steamreforming occurs physically separately therefrom in zone 2. The partialoxidation is carried out using Pt- and Pd-containing catalysts, andcatalysts containing noble metals are used for steam reforming. Acombination of autothermal reforming with a further subsequent steamreforming step is not described.

U.S. Pat. No. 4,415,484 discloses a catalyst for use in an autothermalreforming reactor. The catalyst comprises from 0.01 to 6% of rhodium andfrom 10 to 35% of calcium oxide on a support composed of aluminium oxideand magnesium oxide. According to this document, a typical catalystsystem comprises an iron oxide catalyst for the partial oxidation overabout one third of its length and the rhodium catalyst described overtwo thirds of its length.

EP 1 157 968 A1 describes a single-stage, adiabatically operated processfor the autothermal catalytic steam reforming of hydrocarbons using acatalyst containing noble metals which has been applied to a supportbody. This catalyst catalyses both the partial oxidation and the steamreforming of hydrocarbons.

DE-A 199 55 892 A1 proposes a process for the reforming of hydrocarbons,in particular of diesel, which comprises a noncatalytic step and acatalytic step which take place physically and thermally separately fromone another. In the first step, the hydrocarbon is sent through a burnernozzle and is partially burnt by means of a flame. The fuel gas mixtureis subsequently catalytically reformed in the second step.

DE-A 197 27 841 A1 describes a process and an apparatus for theautothermal reforming of hydrocarbons, in which the fuel is introducedvia a feed device into a two-stage reforming reactor. The resultingreformate is conveyed through a heat exchanger in countercurrent tostarting materials of the reforming reaction conveyed from the outsideinwards so that heat exchange occurs. The fuel fed in via the feeddevice is introduced together with the starting material directly intothe catalyst-containing reaction zone in which combustion and reformingor catalysis are carried out. The reforming reactor contains acatalyst-coated honeycomb body in an upper region and a catalyst-coatedbed in a lower region. It is also possible to use a honeycomb body inplace of the bed.

DE-A 199 47 755 A1 discloses an autothermal reactor for reforminghydrocarbons, which comprises an endothermic reaction zone, anexothermic reaction zone and a downstream cooling zone (quench zone),with the latter being separated off by means of a gas-permeable heatshield. This reactor has a complicated construction and requiresadditional introduction of water into the quench zone and is thereforeexpensive, both to produce and in operation.

A fundamental disadvantage of the known processes for the autothermalreforming of hydrocarbons is the relatively high reaction temperature of650-1000° C. Thus, a fuel gas mixture produced by autothermal reformingof petroleum spirit has a temperature of at least 650° C. at the gasoutlet. The concentration of carbon monoxide in the reformate is in turncoupled to the outlet temperature via the thermodynamic equilibrium.Owing to the high temperatures, the fuel gas has a relatively high COcontent and a low hydrogen content (typical fuel gases at 650° C.contain from about 28 to 36% by volume of hydrogen and from 10 to 15% byvolume of carbon monoxide). The total hydrogen yield and, associatedtherewith, the efficiency of reforming is thus unsatisfactory. Finally,the overall efficiency of a fuel gas system (consisting of gasproduction and PEM stack) is thus also adversely affected. Relativelyhigh hydrogen yields are therefore of critical importance and can beachieved, for example, by means of a reduction in the proportion ofcarbon monoxide in the fuel gas. However, the process temperatures forreforming have to be reduced to achieve this.

A further disadvantage of the existing processes is the fact that, as aresult of the high fuel gas temperatures, expensive and bulky heatexchangers are additionally required in order to cool the fuel gas tothe inlet temperatures of about 450° C. required for the subsequentpurification processes. Apart from the higher costs for the heatexchangers and the greater space requirement, the additional waste heatalso adversely affects the overall efficiency of the fuel gas productionprocess.

It is an object of the present invention to provide an improved processand an improved apparatus for producing fuel gas for fuel cells. Thisobject is achieved according to the invention by provision of theprocess according to Claim 1. Advantageous embodiments of processes andthe apparatus for carrying them out are described in the subsequentclaims.

According to the invention, a smaller space requirement, lower costs anda higher overall efficiency are advantageously achieved. In particular,a process for reforming hydrocarbons which makes it possible to reducethe fuel gas temperatures by about 200° C., for example from 650° C. to450° C., is to be provided in a preferred embodiment. Thehydrogen-containing fuel gas should be able to be passed directly, i.e.without additional cooling, to the subsequent purification stage(s), sothat expensive and bulky heat exchanger systems are dispensed with.

The key part of the novel process for producing fuel gas is a two-stagereforming process. This process consists of the combination ofautothermal reforming (which itself is made up of 2 stages, namelypartial oxidation and steam reforming) with subsequent endothermic steamreforming of hydrocarbons. In the first reaction stage (ATR stage), ahydrogen-containing gas having a temperature above 650° C. is produced.The composition of this gas mixture is set so that it still contains0.1-10% by volume of residual unreacted hydrocarbons. The temperature ofthe fuel gas is reduced to values below 450° C. by means of a subsequentsecond stage in which these residual hydrocarbons are reacted in anendothermic steam reforming reaction (SR stage) as a result of thissecond stage being carried out adiabatically.

The hydrogen yield is thereby increased in two ways: firstly by thefurther conversion in the steam reforming reaction in accordance witheq. (1) and secondly by the fact that as the temperature decreases, theequilibrium of the water gas shift reactionCO+H₂O<=>CO₂+H₂  (4)is moved to the right, i.e. to the side of hydrogen formation. Since theoverall two-stage processes operated adiabatically (i.e. without heatbeing supplied from the outside), the hydrogen-containing fuel gas iscooled to temperatures of about 450° C. and can be passed directly, i.e.without additional heat exchangers, to the subsequent purificationstages.

The proportions of residual hydrocarbons from 0.1 to 10% by volumenecessary for steam reforming can be added to the gas mixture, forexample through nozzles or injectors, before it enters the second stage.Suitable devices for this purpose are, inter alia, conventionalinjection nozzles as are used in motor vehicle engine technology.However, the proportions of hydrocarbons required can also be ensured inthe form of unreacted residues (hydrocarbon “leakage”) by selection ofspecific parameters in the autothermal reforming. For example, theproportion of residual hydrocarbons can be controlled by means of a highspace velocity (typically above 100 000 l/h); such high space velocitiesgenerally result in incomplete conversion of the hydrocarbons.

Furthermore, the residual hydrocarbons in the fuel gas which arenecessary for the subsequent steam reforming can be ensured byconstruction measures on the reactor itself. This can be achieved, forexample, by the use of monolithic catalyst supports having a celldensity below 93 cells/cm² (600 cpsi) or by incorporation of additionalflow channels which have a larger diameter than the remaining flowchannels in the monolith. For example, a monolith having a low celldensity of 62 cells/cm² (400 cpsi) can be used for the first stage(ATR), and a monolith having a high cell density of 186 cells/cm² (1200cpsi) can be used for the second stage (SR).

The water necessary for steam reforming can be added separately ortogether with the hydrocarbon before the second stage. However,depending on the reaction conditions, the external addition of water isnot necessary in many cases, since an appropriate excess of water can beadded in the ATR process in the first stage.

The invention is explained in more detail below with reference to theappended drawings which show:

FIG. 1: Basic structure of the apparatus for the two-stage catalyticreforming of hydrocarbons

FIG. 2: Basic structure of the apparatus for the two-stage catalyticreforming with separate addition of hydrocarbons or water before thesecond stage

FIG. 3: Basic structure of the gas production system of the inventioncomprising two-stage catalytic reforming and a subsequent gaspurification stage (WGS stage or gas separation membrane (GSM))

In a preferred embodiment, the reactor apparatus of the inventioncomprises two stages (ATR stage and SR stage) which contain twomonolithic supports comprising metal or ceramic and are arrangeddirectly after one another. These support bodies can be coated withdifferent catalysts (cf. FIG. 1).

However, it is also possible to use a single monolithic support bodywhich has two segments which are coated with different catalysts.

In a further preferred embodiment (cf. FIG. 2), the two reactors areconnected in series, with a device for introducing hydrocarbon and/oroxygen being installed in the space in between. The introduction can,for example, be effected by means of nozzles or injectors.

FIG. 3 shows the gas production system of the invention comprising thetwo-stage catalytic reforming reactor and a downstream gas purificationstage which can be based on one or more water gas shift stages (e.g.high-temperature WGS, low-temperature WGS or combinations thereof) or ona gas separation membrane (e.g. membranes made of palladium alloys). Inthe case of a subsequent purification of the fuel gas by means of a gasseparation membrane, a further process stage for removing carbonmonoxide to contents below 100 ppm of CO is generally no longernecessary. If the fuel gas is purified in a subsequent water gas shiftstage (WGS stage), a further reduction in the carbon monoxide content tovalues below 100 ppm of CO can, for example, be effected, for example,by means of a PrOx reactor (PrOx=preferential oxidation).

To achieve quick start-up of the overall gas production system, the feedmixture can also be preheated electrically for a short time. The lowthermal mass of the catalysts advantageously leads to fuel gasproduction commencing after only a few seconds.

Catalysts containing noble metals are preferably required for thetwo-stage reforming process of the invention. The catalyst for theautothermal reforming (ATR stage) comprises, for example, a support bodyand a catalyst composition which contains noble metals and has beenapplied in the form of a coating to the geometric surfaces of thesupport body. Preference is given to using platinum and/or rhodium asactive phases; Pd-containing catalysts are also possible. Examples arecatalysts comprising from 0.1 to 5% by weight of platinum on aluminiumoxide and/or from 0.1 to 5% by weight of rhodium on aluminium oxide.Preferred support bodies are monolithic honeycomb bodies comprisingceramic or metal, open-celled ceramic or metal foams, metal sheets orirregularly shaped components. The total thickness of the catalyticcoating is generally in the range from 20 to 200 μm. In the case of amultilayer coating, the catalyst composition can comprise not only alower catalyst layer but also a second, upper catalyst layer, with thetwo layers being able to contain different platinum group metals.

The steam reforming of the residual hydrocarbons in the second stage ofthe reactor (SR stage) is likewise carried out using catalystscontaining noble metals. Catalysts containing at least one of the noblemetals from the group consisting of Au, Pt, Rh, for example, arepossibilities here. Preference is given to using a catalyst comprisingfrom 0.1 to 5% of Rh on aluminium oxide, if desired with additions ofgold and/or platinum. It is here also possible in principle to usemultilayer catalyst coatings, for example coatings comprising Au and Rh;comprising Au, Pt and Rh or comprising Au and Pt.

In general, the noble metals are used in the form of supported catalystsin which the noble metal is finely dispersed on an oxidic supportmaterial. Possible oxidic support materials for the platinum groupmetals are oxides from the group consisting of aluminium oxide, silicondioxide, titanium dioxide and mixed oxides thereof and zeolites.Preference is given to using materials having a specific surface area ofgreater than 10 m²/g in order to make a very fine dispersion of thecatalytically active components on this large surface area possible. Thetechniques for producing such a supported catalyst and for coating aninert support body therewith are known to those skilled in the art.

To increase the thermal stability of the catalyst composition, it canadditionally contain at least one oxide selected from the groupconsisting of boron oxide, bismuth oxide, gallium oxide, oxides of thealkali metals, oxides of the alkaline earth metals, oxides of thetransition elements and oxides of the rare earth metals in aconcentration of up to 40% by weight, based on the total weight of thecatalyst composition. The catalyst layers can additionally containcerium oxide to reduce the formation of carbon deposits and to increasethe sulphur resistance.

The gas production system of the invention can be operated usingaliphatic hydrocarbons (methane, propane, butane, etc.), aromatichydrocarbons (benzene, toluene, xylene, etc.), hydrocarbon mixtures(e.g. natural gas, petroleum spirit, heating oil or diesel oil) oralcohols (e.g. ethanol). Depending on the hydrocarbon used, it can beoperated at steam/carbon ratios S/C of from 0.7 to 5. The air index λ ofthe feed mixture and its preheating temperature are selected so that atemperature in the range from 600 to 800° C., preferably 650° C., isestablished at the outlet of the first ATR stage.

The gas production system proposed or the apparatus can be used forobtaining hydrogen or hydrogen-containing mixtures for mobile andstationary fuel cells.

The following examples illustrate the subject matter of the invention.

EXAMPLE 1

A mixture of isooctane and toluene (each 50% by weight) is reformed bythe process of the invention in a two-stage reactor (comprising an ATRstage and an SR stage, construction as shown in FIG. 1). The reactorinlet temperature of the ATR stage is 400° C., the air stoichiometry (λvalue) is 0.3 and the S/C value is 3. The space velocity (“SV”) of thereaction is set to SV=150 000 l/h, so that incomplete conversion of thehydrocarbons occurs. In steady-state operation, the reformate afterpassing through the first stage contains a proportion of about 5% byvolume of residual hydrocarbons; the temperature of the reformatemixture at the outlet of the ATR stage is 650° C. A monolith having acell density of 62 cells/cm² (400 cpsi) and a volume of 35 cm³ is usedas catalyst for the ATR stage. The catalytic coating comprises arhodium/aluminium oxide supported catalyst and has been applied to thehoneycomb body in a concentration of 150 gram per litre. The coatingconcentration of the rhodium is 1 g/l (=0.67% by weight of Rh).

The reformate is introduced at 650° C. into the second stage (SR stage).A monolith which has 186 cells/cm² (1200 cpsi) and a volume of 140 cm³and has been coated with a rhodium/aluminium oxide supported catalyst isused as catalyst for the SR stage. The coating concentration of thecatalyst is 150 g/l, the coating concentration of the rhodium is 3 g/l(=2% by weight of Rh). The temperature at the outlet from the secondstage is 450° C.

The hydrogen concentration of the reformate is 40% by volume, and the COconcentration is 8% by volume. The reformate produced in this way thushas a high hydrogen concentration and is fed directly into an WGSreactor. In this high-temperature shift stage, the CO content of thefuel gas is reduced further.

EXAMPLE 2

A mixture of isooctane and toluene (each 50% by weight) is reformed bythe process of the invention in a two-stage reactor (comprising an ATRstage and a separate SR stage as shown in FIG. 2). The reactor inlettemperature of the ATR stage is 400° C., the air stoichiometry (λ value)is 0.3 and the S/C value is 3. The space velocity (SV) of the reactionis set to SV 50 000 l/h. A mixture of isooctane/toluene (1:1) isintroduced by means of an injector nozzle located between the tworeactors. The amount introduced is set so that a hydrocarbon content of3% by volume is obtained in the reformate gas upstream of inlet into the(second) SR stage.

A monolith having a cell density of 62 cells/cm² (400 cpsi) and a volumeof 70 cm³ is once again used as catalyst for the ATR stage. It has beencoated with a supported catalyst comprising 0.67% by weight of rhodiumon aluminium oxide. The temperature of the gas mixture at the outlet ofthe ATR stage is 630° C. A monolith which has 1200 cpsi and a volume of140 cm³ and has been coated with a supported catalyst comprising 2% byweight of rhodium on aluminium oxide is used as catalyst for the SRstage. The coating concentration of the catalyst is 150 g/l, and that ofrhodium is 3 g/l. The temperature at the outlet of the SR stage is 440°C. and the hydrogen concentration of the reformate is 40.5% by volume,and the CO concentration is 7.5% by volume. The reformate produced inthis way has a high hydrogen concentration and is fed directly into amembrane reactor (based on a Pd gas separation membrane). In thisreactor, the CO content of the fuel gas is reduced to such an extentthat it can be fed directly into a PEM fuel cell.

COMPARATIVE EXAMPLE CE1

The single-stage standard process for autothermal reforming is employedto demonstrate the improvements achieved by the two-stage process of theinvention.

A mixture of isooctane and toluene (each 50% by weight) is reformed bythe standard process (described in EP 1 157 968 A1, Example 1) in asingle-stage reactor. The reactor inlet temperature of the ATR stage is500° C., the air stoichiometry (lambda value) is 0.3 and the S/C valueis 1.5. The space velocity (SV) of the reaction is set to SV=30 000 l/h.A monolith having a cell density of 62 cells/cm² (400 cpsi) and a volumeof 35 cm³ is used as catalyst for the ATR stage. The catalytic coatingconsists of a rhodium/aluminium oxide supported catalyst and has beenapplied in a concentration of 150 gram per litre to the honeycomb body.The coating concentration of the rhodium is 1 g/l (=0.67% by weight ofRh). The temperature of the reformate mixture leaving the catalyst is680° C. The reformate contains (in addition to nitrogen and carbondioxide) 36% by volume of hydrogen and 12% by volume of carbon monoxide.The reformate produced thus has a lower hydrogen concentration andadditionally has to be cooled to 450° C. by means of a heat exchangerbefore being introduced into the WGS stage. Only then can it be fed intothe high-temperature shift stage of the gas production system. Thesuperiority of the process of the invention can be seen.

1. Process for producing hydrogen-containing fuel gases for fuel cellsby catalytic reforming of hydrocarbons and subsequent gas purification,wherein the catalytic reforming has two successive stages of which thefirst stage comprises autothermal reforming and the second stagecomprises downstream steam reforming at temperatures below 650° C. 2.Process according to claim 1, characterized in that the catalyticreforming is carried out adiabatically and the reformate mixture at theoutlet from the first stage of autothermal reforming has a temperatureof from 650 to 850° C.
 3. Process according to claim 1, characterized inthat the reformate mixture at the outlet from the second stage of steamreforming has a temperature of from 400 to 650° C.
 4. Process accordingto claim 1, characterized in that the reformate mixture at the outlet ofthe autothermal reforming stage has a residual hydrocarbon content offrom 0.5 to 10% by volume.
 5. Process according to claim 1,characterized in that catalysts comprising support bodies to whichsupported catalysts containing noble metals have been applied are usedfor both stages.
 6. Process according to claim 5, characterized in thatone or more noble metals from the group consisting of rhodium, platinumand palladium immobilized on oxidic support materials are used ascatalysts for the autothermal reforming and one or more noble metalsfrom the group consisting of gold, rhodium and platinum immobilized onoxidic support materials are used as catalysts for the steam reforming.7. Process according to claim 1, characterized in that the fuel gasafter the two-stage reforming is passed directly without interpositionof one or more heat exchangers to a gas purification stage.
 8. Processaccording to claim 1, characterized in that the gas purification stagecomprises one or more water gas shift stages or one or more gasseparation membranes.
 9. Apparatus for producing hydrogen-containingfuel gases for fuel cells by catalytic reforming of hydrocarbons andsubsequent gas purification, comprising two successive reactor stagesfor catalytic reforming, with the first reactor stage having at leastone catalyst for autothermal reforming and the second reactor stagehaving at least one catalyst for steam reforming and no heat exchangerbeing installed between the second reactor stage and the gaspurification stage.
 10. A mobile or stationary fuel cell in which theprocess of claim 1 is used.
 11. A mobile or stationary fuel cell inwhich the apparatus of claim 9 is used.